MySql 使用内连接和联合不同

Question

How to I add a inner join to the following union distinct query?

select cn_to from connections a where cn_from = '111'
union distinct
select cn_from from connections b where cn_to = '111'
inner join names on connections.cn_to = names.id

The tables are names and connections . The first query works fine. Things fail at the inner join part. Can you please help?

Table creates and inserts are here http://sqlfiddle.com/#!9/ca27bf/1

I'm trying to get results like this

Name 2
Name 3
Name 4
Name 5
Name 6
Name 7
Name Odd

Answer 1

The correct syntax would be:

select c.cn_to
from connections c
where c.cn_from = '111'
union distinct
select c.cn_from
from connections c join
     names n
     on c.cn_to = n.id
where c.cn_to = '111';

WHERE and FROM are clauses in the SELECT statement. JOIN is an operation only understood in the FROM clause . So your clauses are misplaced.

This assumes that your database supports union distinct .

Answer 2

You need to write the UNION query as a derived table and then JOIN that to the names table:

SELECT cn, names.name
FROM (select cn_to AS cn from connections a where cn_from = '111'
union distinct
select cn_from from connections b where cn_to = '111') c
join names on c.cn = names.id

Output:

cn      name
222     Name 2
333     Name 3
444     Name 4
555     Name 5
666     Name 6
777     Name 7
121     Name Odd

Demo on SQLFiddle

Answer 3

what you want to select cn_to or name?? if you want names for cn_from = '111' or cn_to = '111':

result: name Name 2 Name 3 Name 4 Name 5 Name 1 Name 1 Name 1 Name 1 link: http://sqlfiddle.com/#!9/ca27bf/13